NREL 5MW_Onshore: Using Bmodes & ModeShapePolyfitting.xls

Dear Jason,

I’m trying to learn how to use Bmodes & ModeShapePolyfitting.xls and trying to validate my results with the Mode Shape coefficient terms from “NRELOffshrBsline5MW_Onshore_ElastoDyn_Tower” file. The resultant mode shapes (except for the first modes) that I have got are not matching with “NRELOffshrBsline5MW_Onshore_ElastoDyn_Tower” viz.
---------------------- TOWER FORE-AFT MODE SHAPES ------------------------------
0.7004 TwFAM1Sh(2) - Mode 1, coefficient of x^2 term
2.1963 TwFAM1Sh(3) - , coefficient of x^3 term
-5.6202 TwFAM1Sh(4) - , coefficient of x^4 term
6.2275 TwFAM1Sh(5) - , coefficient of x^5 term
-2.504 TwFAM1Sh(6) - , coefficient of x^6 term
-70.5319 TwFAM2Sh(2) - Mode 2, coefficient of x^2 term
-63.7623 TwFAM2Sh(3) - , coefficient of x^3 term
289.737 TwFAM2Sh(4) - , coefficient of x^4 term
-176.513 TwFAM2Sh(5) - , coefficient of x^5 term
22.0706 TwFAM2Sh(6) - , coefficient of x^6 term
---------------------- TOWER SIDE-TO-SIDE MODE SHAPES --------------------------
1.385 TwSSM1Sh(2) - Mode 1, coefficient of x^2 term
-1.7684 TwSSM1Sh(3) - , coefficient of x^3 term
3.0871 TwSSM1Sh(4) - , coefficient of x^4 term
-2.2395 TwSSM1Sh(5) - , coefficient of x^5 term
0.5357 TwSSM1Sh(6) - , coefficient of x^6 term
-121.21 TwSSM2Sh(2) - Mode 2, coefficient of x^2 term
184.415 TwSSM2Sh(3) - , coefficient of x^3 term
-224.904 TwSSM2Sh(4) - , coefficient of x^4 term
298.536 TwSSM2Sh(5) - , coefficient of x^5 term
-135.838 TwSSM2Sh(6) - , coefficient of x^6 term

Here’s what I did,

  1. Using Bmodes
    First I ran Bmodes file (PFA) file and got the output file.
    As you have mentioned a couple of times before in this forum, I changed the following properties in the “tower_secs” file.
    str_tw is set to zero
    tw_iner is set to zero
    cg_offst is set to zero
    sc_offst is set to zero
    tc_offst is set to zero
    edge_iner & flp_iner is set to very small number
    tor_stff & axial_stff is set to very high number

  2. Inferring the Associated Modes
    From the output file, this is what I concluded:
    The first mode is corresponding to SS Mode 1(FA1) ,
    Second mode is corresponding to FA Mode 1 (SS1),
    Third mode is corresponding to SS Mode 2(FA2) ,
    Fourth mode is corresponding to FA Mode 2 (SS2).

The following attachment has the Input file, Tower_secs file and the Results file.
NREL5MW_Land Input file,tower_secs and Results.txt (67.6 KB)

  1. Using ModeShapePolyfitting.xls
    Then I copied the respective displacements to “ModeShapePolyfitting.xls” and tried to get the polynomial coefficients. I set the “slope at the bottom” as 0 since the model is cantilevered at the base. The “Scaling factor of y” was linked to the suggested value. I then selected the 6th order polynomials as suggested by “Normalized Projection Method”. An example for SS Mode 2 is shown in the attached image

  1. I have attached the plot of the four modes, comparing my results (5MWLand estimate) and the “NRELOffshrBsline5MW_Onshore_ElastoDyn_Tower” file ( 5MWLand) coefficients.

Any help in this regard will be highly appreciated.

Thanks & Regards,
Edwin

Dear Edwin,

Your overall approach sounds correct.

I would not expect a perfect match to the mode shapes in the “NRELOffshrBsline5MW_Onshore_ElastoDyn_Tower.dat” file because I derived those mode shapes using ADAMS (with a flexible rotor) in place of BModes, as discussed in the following forum topics: http://forums.nrel.gov/t/tower-eigenfrequencies-of-nrel-5mw-turbine/517/1 (see my posted dated Tue Aug 28, 2012).

Please note that (FA1) is swapped with (SS1) and (FA2) is swapped with (SS2) in this excerpt from your post:

Best regards,

Dear Jason,

Thanks for the reply.
You are right, It was a typo in the post. But the figures that I had attached are correct.
The associated frequencies are,
SS1: 0.32950 Hz
FA1: 0.33271 Hz
SS2: 1.8779 Hz
FA2: 2.2802 Hz

My question now is, can I use the Estimated Mode Shapes that I calculated using Bmodes instead of the one given in “NRELOffshrBsline5MW_Onshore_ElastoDyn_Tower.dat”?. Would it significantly change/affect the results (response of wind turbine) that I get using FAST simulations?

Thanks & Regards,
Edwin

Dear Edwin,

Yes, you should be able to use your new mode shapes. The only change that is significant is for the second side-to-side tower bending mode. I suspect you’ll see a minor change in the response to excitations of this mode. The only way to see the influence is to try it. Please post the results if you choose to do so…I’d be interesting in seeing the effect.

Best regards,

Hi everybody,

I am working on my master’s thesis, where the objective is to develop a floating support structure for a tower with an elliptical cross section.

My intention is to use Bmodes to calculate the modeshape coefficients and then use the modeshapepolyfitting afterwards to get the coefficients. However, in the manual to Bmodes it is written that the tower has to be axisymmetric, which it of course is not with an elliptical cross section. In ElastoDyn for the tower it is possible to specify bending stiffnesses and mode shapes for TwFASTIF (fore-aft) and TwSSStif (side-side), so FAST should be capable of calculating the elliptical tower cross section. My question now is, which software I am recommended to use instead of Bmodes?

Another question:
In his report on the MIT tension leg platform, Denis Matha used ADAMS to get the mode shapes (p. 34). Is this generally necessary to run the structure in ADAMS if I change some of the dimensions of the support structure or can I use FAST8.16?

Dear Jacob,

Actually, BModes can be applied to towers that are not axisymmetric. I’m not sure why the draft manual available on our website says otherwise, because in the same manual it talks about different stiffness in the fore-aft and side-to-side directions: nrel.gov/docs/fy06osti/39133.pdf. Regardless, we routinely use BModes for cases where the fore-aft stiffness differs from the side-to-side tower stiffness.

In Denis Matha’s MS thesis-turned NREL report: nrel.gov/docs/fy10osti/45891.pdf, ADAMS was not used in place of FAST. Instead, Denis used ADAMS in place of BModes to calculate the tower mode shapes needed for FAST, as discussed in my post dated Apr 17, 2015 above and in the forum topics linked there.

Best regards,

Dear Jason,

Thanks a lot.

Do you know of any reason why ADAMS was used instead of Bmodes? I am thinking of using Bmodes rather than ADAMS for a floating structure to calculate the modeshapes as was done for the barge in the report “Modal Dynamics of Large Wind turbines with different support structures”.

Dear Jacob,

The reasoning why ADAMS was used in place of BModes at the time is explained in the forum topics linked in my Apr 17, 2015 post above. Basically, (1) the 6 DOFs of the platform were not available in BModes at the time and (2) the flexibility of the rotor has some influence on the tower mode shapes, and this flexibility can be included in ADAMS, but not BModes.

Best regards,

Dear Jason,

I have designed an elliptical tapered cross section for the wind turbine tower to support the NREL 5MW reference wind turbine. I have chosen the dimensions of the ellipse so that the 1st SS natural frequency should be 0,18Hz and the 1st FA natural frequency should be 0,23 Hz. I have used BmodesJJ for verification of the calculated eigenfrequencies. I get approximately 0,23 Hz for the first two natural frequencies and not 0,18 Hz as I expected. For an arbitrary cantilever beam with different area moment of inertia, I_xx and I_yy, I would expect to get natural frequencies corresponding to the value of the area moment of inertia of that axis, but this is not the case here. Can you maybe help? I attached my input files and the result file for BmodesJJ.
JKP_elliptical_tower.rar (5.96 KB)

Dear Jacob,

According to the BModes documentation: nwtc.nrel.gov/system/files/BModes.pdf, the tower in BModes is assumed to be axisymmetric, so, edge_stff is set equal to flp_stff and edge_iner is set equal to flp_iner even if they are specified uniquely in the BModes input file.

My guess is you could specify the beam as a blade (with no rotational speed) to see the effect you want.

Best regards,

Dear Jason,

That worked and I now get the first two natural frequencies almost identical to my own calculations.

Thanks.